The invention relates to an image processing method for noise reduction in a sequence of noisy images representing a threadlike structure on a background. The invention particularly relates to noise reduction method applied to a sequence of medical X-ray fluoroscopy images representing a catheter guide-wire. The invention also relates to a system for carrying out this method and to an examination apparatus having means for image processing. The invention is applied to the medical imaging.
An image processing system for filtering X-ray fluoroscopic image sequence is already known of a U.S. Pat. No. 5,809,105. This document discloses means for acquiring one image by passing X-rays through a subject, said image comprising an array of pixel values, means for receiving the image and producing a mask therefrom which distinguishes the pixel values that correspond to structural features in the image from the pixel values that correspond to background and means being responsive to the mask for selectively filtering the pixel values in the image that correspond to background. The means for filtering the background pixel values include a Poisson filter which is spatial filter. So, the mask is used to select the background pixels for filtering the background noise while enabling the structural pixels to pass unfiltered to the display.
This known method does not provide means to detect object motion in an image sequence. By use of the method it may be difficult to filter the background by applying the mask without degradation of said structural features, especially if they represent a very thin threadlike structure. The mask may cover up said threadlike structure when a non-detected swift motion occurs. Besides, this known method does not provide means to eliminate phantom artifacts. Furthermore, this known method does not provide means for noise filtering the object pixels themselves. This is a drawback because the boundaries of a thin threadlike structure may be blurred by noise, which makes said threadlike structure difficult to localize, whereas the central points of the threadlike structure may be sputtering due to noise, which makes this threadlike structure difficult to visualize, even when the background has been noise filtered. Moreover, the known method does not permit enhancement of the structural features with respect to the background. This is a drawback when the thin threadlike structure presents little contrast with respect to a noisy cluttered background.
In cardiology, a sequence of images may be formed of images representing blood vessels called arteriogram images used to visualize in real time medical procedures of introducing a catheter in a vessel. Such medical procedures deeply rely on the correct visibility of a metallic guide-wire, which is introduced in the vessel for guiding the catheter. These fluoroscopic digital images are noisy because they are formed with a low level of X-rays. It is very important to improve considerably the guide-wire visualization in image sequences formed in the X-ray fluoroscopic mode in order to help the practitioner direct the catheter in the arteries thereby avoiding damage said arteries during this operation.
The invention aims at providing an image processing method to be applied to a noisy image sequence representing a threadlike structure on a background, at improving the background noise reduction without degradation of said threadlike structure, at eliminating the phantom artifacts and at noise filtering and enhancing the threadlike structure in real time, for example at a rate of about 25 images per second.
These aims are reached by an image processing method for noise filtering an image sequence representing a threadlike structure on a background, including an acquisition of point data in first and second sequence images and an estimation of the corresponding filtered point data for constructing a filtered second image, the point data being the intensities associated to co-ordinates of image points, said method comprising, performed in each sequence image, steps of: extracting the threadlike structure points, forming strings from said extracted points, temporally filtering the data of the points located outside the strings denoted background points, spatially filtering the data of the string points, and constructing the filtered second image data by performing an insertion of the spatially filtered data of the string points into the temporally filtered data of the background points.
Advantages of this method are that the object of interest localization is much more accurate, while the processing method is carried out in real time.
A system to carry out said method includes a computer program product comprising a set of instruction for carrying out a method of image processing for noise filtering an image sequence representing a threadlike structure on a background; and an examination apparatus with a system and means for carrying out the processing method as claimed in claim 6, including an enhancement operation performed on spatially filtered string point data, comprising steps of: selecting string points [A(i,j)] using the list of the control signal [St(i, j)], enhancing the string point data with respect to their local environment, and constructing a filtered second image data by performing an insertion of the enhanced spatially filtered data of the string points into the temporally filtered data of the background points, controlled by the binary control signal.